This invention relates to polyurea or polyurea-polyurethane polymers prepared in a reaction injection molding (RIM) process.
Polyurethane elastomers are increasingly being made from fast-reacting precursor materials in a reaction injection molding (RIM) process. The RIM process has the advantages of cycle times of the order of three minutes or less and the ability to handle highly reactive precursor materials which cannot be processed using conventional casting processes. The use of these precursor materials is especially advantageous as they give rise to desirable properties in the product.
Recently, the hydroxyl-containing materials used in RIM systems have been gradually replaced with primary or secondary amine-containing materials. These amines react with a polyisocyanate to form a polyurea or polyurea-polyurethane polymer. The urea groups have been related to improved thermal properties, especially improved heat distortion. This property is particularly important in making automobile body parts, which is the primary application of the RIM process. The process used for painting these parts requires baking at a temperature of up to 400.degree. F. Polyurea and polyurea-polyurethane polymers withstand these temperatures with much less distortion than polyurethanes.
A major disadvantage of the polyurea and polyurea-polyurethane systems is that, in order to obtain good physical properties, it is necessary to employ very high mold temperatures, such as about 260.degree.-325.degree. F. At least three problems are associated with the use of such high mold temperatures. First, the energy consumption is much greater than that required at lower mold temperatures. Second, the higher temperatures make the parts much harder to handle at demold. Since the parts must usually be demolded by hand, and the mold treated at least occasionally between successive moldings, the higher temperatures make these operations less comfortable to work in and increase the risk of significant injury. Third, at these high temperatures, the best performing internal mold release agents, the metal carboxylates, tend to perform more poorly.
For these reasons, it would be desirable to provide a polyurea or polyurea-polyurethane system which is processable using relatively low mold temperatures, but which provides a polymer having excellent physical properties.